This invention relates to improvements in the polymerization of low molecular weight cyclopolydihalophosphazenes to higher molecular weight linear polydihalophosphazenes. More particularly, the invention relates to the use of a catalyst which comprises the reaction product of trihalides of boron and oxygenated phosphorus compounds in order to improve the polymerization of cyclopolydihalophosphazenes while decreasing or entirely eliminating the formation of gel and by permitting better control of the molecular weight and other physical properties of the resultant polymer.
The polymerization of low molecular weight cyclic polyhalophosphazenes such as (NPCl.sub.2).sub.3 or (NPCl.sub.2).sub.4 to high molecular weight polydihalophosphazene polymer is known in the phosphazene art.
Early polymerization processes involved the uncatalyzed thermal polymerization of such cyclic polydihalophosphazenes utilizing either bulk or batch processes. Such early polymerization processes are described in U.S. Pat. No. 3,370,020 to Allcock et al issued Feb. 20, 1968 and U.S. Pat. No. 3,515,688 to Rose issued June 2, 1970.
While these early polymerization processes provided for the conversion of cyclic polydihalophosphazenes to linear high molecular weight polydihalophosphazene polymer, they exhibited a number of serious disadvantages. Thus, for example, these early processes required relatively high polymerization temperatures, e.g., 200.degree. to 300.degree. C. and lengthy polymerization times (i.e., slow conversion rates). Additionally and more seriously, these processes often resulted in the formation of gelled or partially gelled polymers which were insoluble in conventional solvents and could not readily be derivatized to produce useful polymeric products such as those described in the aforementioned patents. Moreover, control of the molecular weight of the finished polymer was extremely difficult using these early processes.
Recent attempts to overcome or at least minimize the disadvantages inherent in these early processes have involved the investigation of catalyzed polymerization systems. Thus, U.S. Pat. No. 4,005,171 to Reynard et al describes the polymerization of (NPCl.sub.2).sub.3 or (NPCl.sub.2).sub.4 or mixtures thereof using as catalysts metal or organometal salts of very strong acids or strong acids and derivatives of halocyclicphosphazenes; U.S. Pat. No. 4,123,503 to Snyder et al describes the polymerization of (NPCl.sub.2).sub.3 or (NPCl.sub.2).sub.4 or mixtures thereof using Lewis acid type compounds of Al or B such as boron trihalides, alkyl aluminum and alkyl aluminum halides; U.S. Pat. No. 4,137,330 to Prichard et al describes the polymerization of (NPCl.sub.2).sub.n in which n is from 3 to 9 using an inorganic salt catalyst, preferably a metal halide such as CrCl.sub.2, NiCl.sub.2, MgCl.sub.2 etc.; U.S. Pat. No. 4,110,421 and 4,124,567 to Dieck et al describes the polymerization of (NPCl.sub.2).sub.y is 3 or 4 or mixtures thereof using as a catalyst a metal alkoxide of the formula M(OR').sub.x and copending application Ser. No. 898,007 to Snyder et al filed April 20, 1978 commonly assigned to applicants' assignee herein describes the polymerization of (NPHal.sub.2).sub.n in which Hal is halogen and n is an integer of 3 to 7 using as a catalyst a Ziegler type catalyst such as a compound of Tr, Zr, HF, V etc. with or without a Boron or Aluminum compound as a cocatalyst.
While the above catalyzed polymerization processes are advantageous in many respects, they also exhibit certain disadvantages. Thus, in general, the polymerization processes and catalyst systems described in the aforementioned patents and copending application do not provide the entire spectrum of factors such as relatively low polymerization temperatures, rapid conversion rate, control of molecular weight of the linear polymers and ease of derivatization and processability of linear polymer (e.g., solubility, etc.) which is desirable. Furthermore, many of these processes do not provide the uniformity and consistency of properties desired in the linear polymer.
In contrast, the polymerization process and catalyst composition of the present invention while obviating or greatly minimizing the disadvantages encountered in the early processes provides the desired balance of the above factors and additionally provides uniformity and consistency in the properties of the linear polymer.